Problem: $-4rs + rt - 9r - 2 = 9s + 4$ Solve for $r$.
Combine constant terms on the right. $-4rs + rt - 9r - {2} = 9s + {4}$ $-4rs + rt - 9r = 9s + {6}$ Notice that all the terms on the left-hand side of the equation have $r$ in them. $-4{r}s + 1{r}t - 9{r} = 9s + 6$ Factor out the $r$ ${r} \cdot \left( -4s + t - 9 \right) = 9s + 6$ Isolate the $r$ $r \cdot \left( -{4s + t - 9} \right) = 9s + 6$ $r = \dfrac{ 9s + 6 }{ -{4s + t - 9} }$